digital voltmeter question

Since it works with the PC supply, your meters do not need an isolated power source so that's not it.  I realize the top photo shows +14.6V DC and the connections look simple enough but could the HO supply be in Reverse supplying -14.6V DC to power the meters?  Or could the HO supply be of the pulsed-output PWM type where the average voltage is indeed 14.6V DC but the pulsing bothers the meter electronics?

I don't think I have ever seen an HO power pack that outputs a pure-ish DC. Many are just a fullwave CT/bridge rectifier and a rheostat, the better ones are PWM. The claim is that the higher peak-to-average ratio of the PWM/ripple pulse output permits better low speed control than a pure low-voltage DC.

Don't know if that applies in this case or not.

I'm pretty sure the polarity is right because when I slide the reverse switch the Fluke meter reads negative and the meters still don't light. The specs say I only need a separate supply for over 30v. I planned on using the PC supply for all my buildings, so no problem there. I now have an extra meter because it was easier to just pay for the replacement they sent me rather than send it back. I was just looking for the reason why. Thanks for the input.

So in the spirit of a high-school-science-project, can't you power the meters with the PC supply...but measure the HO supply output?  They look to be the 3-wire meter style (red, black, yellow) rather than the 2-wire style that only measures its power supply.  This might resolve question if HO supply is not suitably smooth for use as the power source for these meters.

Stan,

i could, but the HO transformer is just a bench tester. I planned all along to use the PC supply on the layout. That's what got me into trouble. 

I'm missing something.   I thought we're trying to understand why the meters don't work when powered by the HO supply.  Has this been resolved? 

I'm sorry, I missed the point. I'll give it a try. 

OK, Stan,

   The meter will read the HO transformer voltage when powered by the PC power supply, so the HO DC voltage is obviously not suitable for powering the meters.

Just for S and giggles, if you have a big capacitor laying about, say 470uF, stick it across the supply voltage from the transformer.  Also try with a small cap as well, say 1uF.  My guess is the transformer is either unfiltered, or not filtered enough for the meter.  Computer power supplies, on the other hand, tend to put out  very well filtered power.  

JGL

John H posted:

OK, Stan,

   The meter will read the HO transformer voltage when powered by the PC power supply, so the HO DC voltage is obviously not suitable for powering the meters.

Most of these meters (Asian variety)require a separate power source. The 3 wire (read/black leads) power the meter, the 2 wires measure load

revisit the listing from your purchase and try to determine which type you received. Attached is one example of a two wiring diagrams.

Do these require an external shunt for the amperage?

Put a bulb or a load into the connections to find the way to wire it. use the PC supply.

I have what is in your photos. I won't be over 30v so I'm good to go. 

John,

     I put a 220uF capacitor in like you suggested and the meter works with the HO transformer. However, now I get 22v instead of 14v at maximum. Why is that?

Simple electronics.  The capacitor is charging to the peaks of the rectified current.  Also, the DC meter probably doesn't read the unfiltered DC all that accurately.

I get the same readings on my Fluke, I just didn't take time to post a picture. If I put a capacitor across the terminals on the HO transformer to continue using it on the bench, what size would cover most of the usage?

The more current you draw, the larger the capacitor needs to be.  You have to decide what is acceptable ripple voltage for your usage.

Take a look at this graph.  It's probably sufficient for your needs.  You'll see that without an active regulator, you need a LOT of capacitance to smooth the ripple if you draw any significant current.   This is why a bench power supply typically uses active regulation, the regulator knocks out the residual ripple without a huge capacitor.

Thank you, John. I guess I'm ignorant enough to not know what my acceptable ripple voltage is. I like having the variable voltage to check lighting in buildings and accessories and running engines to lube and clean wheels. It looks like I would be off the graph except for LEDs. I have an extra PC supply, but only three voltage choices with that.

For most purposes, the ripple will have little effect.  I'd probably round up a 4700uf 35V capacitor and declare myself the winner.  My guess is you'll never experience an issue with ripple unless you do something much more critical with that supply.

I'll get the bigger cap; I only have a 1000uf. Also, while I have your ear, what would be a good assortment range for a resistor kit? I only  have a few individual ones that I got when Radio Shack closed around here.

Well, I find, other than my work with electronic designs, that values from around 22 ohms to maybe 4.7kohms are by far the most common use items for me.  I like to have all of those in 1/4W ratings and then some of the lower value ones, less than around 150 ohms, in 1W and maybe 2W sizes.  Of course, I have the higher values as well, but with the typical train applications, you won't often reach for them.

Remember, when you're considering kits, being able to find the values after you have them all in hand is very important.  You'll want a set of drawers with graduated values to make locating what you need easy.

You'll also want a similar set of capacitors, though unless you're doing circuit design, the smaller values can be a much smaller list.

I tend to buy when I need them, as time goes by, I get a good assortment of values I most often use.  My uses are a bit more complicated as I need various size surface mount components as well.

I tend to like picking up an assortment pack for both resistors and capacitors to make a starter kit.  Radio Shack sells/sold a package with a large assortment of resistors for like 8 bucks that was a good starting place.  You can also find countless options on the auction site with anything from 300 to 2500 pieces starting around $2.  

Caps are equally available for low prices, though you may want to stick with the higher value ones to start.  Look for an electrolytic assortment, as these will have more useful values for the sorts of things we do with trains.  Also look for ones that are rated for 35V or more.  I think the last assortment I picked up off the auction site was 200 pieces for about 10 bucks, though not all of the values in that assortment were 35v+  some were 25v or less.  

I have also used the plastic drawer method to sort parts, and it works very, very well to make things easy to find.  Lately all the equipment from my father is packed up and inaccessible, so I'm using a couple small plastic boxes with dividers.  Here I  just have one big section for 'Resistors' so I've found it useful to write the values on the cardboard securing each value.  I can manage the color code well enough, but still have to think about it, where as having it written down it is fairly quick to find what you are looking for.  It may also help to keep a copy of the color code with your resistors until you get the hang of it.  

JGL

For storage, I got some of these a few years ago at Harbor Freight. They are quite nice, pretty sturdy and also handy for small parts like resistors and capacitors or train parts. The parts bins have the good latches on them too. You can put your meter or other tools and larger items in the top tray. A few labels and you know right where everything is. One of HF's better items, IMO.

HF organizer with 4 parts containers

Anyway, just a thought, something I really liked and thought others might like as well.

I use a lot of these for my parts.

20 Bin Portable Parts Storage Case

Very handy and I can mix-n-match the size of compartments between the cases.

You sure can get a lot of information when you hijack your own thread! Thanks guys. I have tried the color chart method; even using a magnifier I was wrong, so I definitely need organization.